NOx and other harmful substances are contained in the exhaust gas of an internal combustion engine. It is known that a NOx catalyst for purifying NOx in the exhaust gas may be provided in the exhaust system of the internal combustion engine to reduce discharge of these harmful substances. In this technology, when a storage-reduction type NOx catalyst is provided, the purification performance decreases as the amount of stored NOx increases, and therefore, a reducer is supplied to the storage-reduction type NOx catalyst to reduce and release the NOx stored in the catalyst (hereinafter referred to as a “NOx reduction treatment”).
Furthermore, a reducer is sometimes supplied to the NOx catalyst in addition to raising the bed temperature of the NOx catalyst in order to counteract SOx poisoning in which SOx in the exhaust gas stored in the NOx catalyst results in a decreased purification performance (hereinafter referred to as a “SOx regeneration treatment”).
Meanwhile, particulate matter (PM) having carbon as a main component is also contained in the exhaust gas of an internal combustion engine. In a known technology for preventing emission of the particulate matter into the atmosphere, a particulate filter (hereafter referred to as a “filter”) is provided in the exhaust system of the internal combustion engine to trap the particulate matter.
In this filter, as the accumulated amount of trapped particulate matter increases, the filter is clogged, causing an increase in back pressure for the gas exhaust and a decrease in the engine performance. As a countermeasure, the exhaust purification performance of the filter can be regenerated by raising the temperature of the exhaust gas that is introduced into the filter such that the temperature of the filter rises and the trapped particulate matter is removed through oxidation (hereafter referred to as “PM regeneration treatment”).
As a method of raising the temperature of the exhaust gas that is introduced into the filter during this PM regeneration treatment, an oxidation catalyst capable of oxidation is disposed on the upstream side of the filter, and during PM regeneration treatment, a reducer is supplied to the oxidation catalyst, causing an oxidation reaction in the oxidation catalyst such that the exhaust gas temperature on the upstream side of the filter rises.
It is known that at this time, the flow rate of the exhaust gas introduced into the exhaust purification device is preferably suppressed. The suppression is performed to prevent the supplied reducer from contacting with, and be oxidized by, the high-temperature exhaust gas, as a result of which the reducer can no longer be used for the oxidation reaction in the storage-reduction type NOx catalyst or in the oxidation catalyst, when a reducer is supplied to an exhaust purification device such as a NOx catalyst or filter to regenerate the purification performance.
Japanese Patent Application Publication JP-A-2003-106142 and Japanese Patent Application Publication JP-A-2003-74328 propose a technology in which an exhaust purification system (to be referred to hereafter as an exhaust purification device, and when a control system thereof is included, an “exhaust purification system”) is provided with a plurality of branch passages, and an exhaust purification device is disposed in each branch passage. In this technology, the flow rate of exhaust gas introduced into one of the exhaust purification devices is suppressed to a predetermined rate by a valve that is capable of varying the passage sectional area, and fuel serving as a reducer is supplied to the exhaust purification device into which exhaust gas is introduced at the suppressed flow rate. In so doing, the supplied fuel can be used efficiently to regenerate the purification performance of the exhaust purification device, and adverse effects on the operating performance of the internal combustion engine are suppressed.
In relation to this technology, Japanese Patent Application Publication JP-A-HEI7-102947 discloses a technology employed in a structure having a plurality of branch passages and an exhaust purification device disposed in each branch passage. In this technology, an exhaust throttle valve in one of the branch passages is closed, and a reducer is supplied when the flow rate of the exhaust gas has decreased. Further, Japanese Patent Publication No. 2947021 discloses a technology employed in a structure having a plurality of branch passages and a NOx catalyst disposed in each branch passage. In this technology, exhaust gas is led to each branch passage alternately by switching a switch valve. Japanese Patent Publication No. 2727906 also discloses a technology employed in a structure having a plurality of branch passages and a NOx catalyst disposed in each branch passage. In this technology, the flow rate of the exhaust gas that flows through one of the branch passages is reduced, and when the flow rate of the exhaust gas has decreased, a rich spike is implemented.
However, in these technologies, there is still room for improvement in ensuring that the fuel serving as a reducer is supplied reliably to the entire exhaust purification device so that regeneration of the purification performance can be performed efficiently.
An object of the present invention is to provide a technology for regenerating the purification performance of an exhaust purification device more reliably or more efficiently in an exhaust purification system combining a plurality of branch passages bifurcating from an exhaust passage and an exhaust purification device provided in each branch passage.